Our objectives are to characterize poorly understood mechanistic processes that control sorption of Ahreceptor ligands (AhRLs) to mineral surfaces, and then use these well-defined systems to probe the bioavailability of sorbed AhRLs to bacteria and mammals. Since AhRLs tend to have exceptionally low water solubilities, these highly toxic compounds exist in the environment predominantly as sorbed species. We argue that clay minerals are a major, largely unrecognized sink for AhRLs in the environment. Our preliminary studies and the literature suggest that clays may play a nearly equal or even dominant role to that of organic matter in the immobilization of dioxins in soils. Our proposal has four specific aims: 1) To measure sorption and sequestration of key AhRLs to clay minerals and to characterize the geochemical controls on that sorption, 2) To determine the molecular mechanisms of such sorption through integrating bulk results with spectroscopy and molecular simulation studies, thereby promoting more rational long-term Superfund site stewardship, 3) To identify microbial genetic and functional responses to minerals themselves and to mineral-adsorbed AhRLs in order to elucidate key mechanisms of microbial interaction with these adsorbed compounds, and 4) To quantify the microbial functionality enabling bioavailability and biodegradation of mineral-sorbed AhRLs by using fluorescent markers that are expressed with key microbial responses. Thus our proposal supports the mission of the Superfund Basic Research and Training Program because in Objective 1 we will work to "identify and quantify chemical forms of the contaminants," in Objectives 1-2 we will assess "the physical, chemical and biological processes affecting chemicals in environmental media," and in Objectives 3-4 we will link this work directly to bioavailability of Superfund chemicals to bacteria and to mammals. Relevance of this research to public health: Many toxic organic contaminants are so insoluble in water that the main route of human exposure is through ingestion of adsorbed contaminants. We propose to characterize adsorption of dioxins and related compounds to clay minerals, and then use both bulk and molecular methods to understand the effects of adsorption on the toxicity of the compound (to mice) and the biodegradation of the compound (by soil bacteria), which is needed in remediation and risk assessment.